In the human eye, the tear film covering the ocular surfaces is composed of three layers. The innermost layer in contact with the ocular surface is the mucus layer. The mucus layer is comprised of many mucins. The middle layer comprising the bulk of the tear film is the aqueous layer. The aqueous layer is important in that it provides a protective layer and lubrication to prevent dryness of the eye. Dryness of the eye can cause symptoms such as itchiness, burning, and irritation, which can result in discomfort. The outermost layer is comprised of many lipids known as “meibum” or “sebum.” This outermost lipid layer is very thin, typically less than 250 nm in thickness. The lipid layer provides a protective coating over the aqueous and mucus layers to limit the rate at which these underlying layers evaporate. A higher rate of evaporation of the aqueous layer can cause dryness of the eye. Thus, if the lipid layer is not sufficient to limit the rate of evaporation of the aqueous layer, dryness of the eye may result. The lipid layer also lubricates the eyelid during blinking, which prevents dry eye. Dryness of the eye is a recognized ocular disease, which is generally known as “dry eye.” If the lipid layer can be improved, the rate of evaporation is decreased, lubrication is improved, and partial or complete relief of the dry eye state is achieved.
The sebum that forms the outermost lipid layer is secreted by meibomian glands 10 of the eye, as illustrated in FIGS. 1 and 2 of this application. The meibomian glands are enlarged, specialized sebaceous-type glands (hence, the use of “sebum” to describe the secretion) located on both the upper eyelid 12 and lower eyelid 14. The meibomian glands contain orifices 16 that are designed to discharge lipid secretions onto the lid margins, thus forming the lipid layer of the tear film as the mammal blinks and spreads the lipid secretion. The typical human upper eyelid 12 has about twenty five (25) meibomian glands and the lower eyelid 14 has about twenty (20) meibomian glands, which are somewhat larger than those located in the upper lid. Blinking and the squeezing action of the muscle of Riolan surrounding the meibomian glands 10 are thought to be the primary mechanism to open the orifice for the release of secretion from the meibomian gland 10. The meibomian gland orifices 16 open on the lid margin usually along the mucocutaneous junction also known as the gray line. The meibomian gland orifices 16 are assumed to open with blinking and release minute amounts of sebum secretions onto the lid margin and then into the inferior tear meniscus. The lipid “sebum” in the tear meniscus is spread upward and over the tear film of the open eye by the upward blink action. Blinking causes the upper lid 12 to pull a sheet of the lipids secreted by the meibomian glands 10 over the other two layers of the tear film, thus forming a type of protective coating which limits the rate at which the underlying layers evaporate. If the lipid secretions are optimal, and adequate lipid layer is maintained at the air interface, evaporation is minimized and dry eye states are prevented. If the lipid secretions are inadequate, the lipid layer is not adequate to minimize evaporation with resulting rapid evaporation leading to dry eye states. Thus, a defective lipid layer or an insufficient quantity of such lipids can result in accelerated evaporation of the aqueous layer which, in turn, causes symptoms such as itchiness, burning, irritation, and dryness, which are collectively referred to as “dry eye.”
Various treatment modalities have been developed to treat the dry eye condition. These modalities include drops, which are intended to replicate and replace the natural aqueous tear film and pharmaceuticals which are intended to stimulate the tear producing cells. For example, eye drops such as Refresh Endura™, Soothe™, and Systane™ brand eye drops are designed to closely replicate the naturally occurring healthy tear film. However, their use and administration are merely a treatment of symptoms and not of the underlying cause. Further, the use of aqueous drops is generally for an indefinite length of time and consequently, extended use can become burdensome and costly.
Pharmaceutical modalities, such as the use of tetracycline, have also been suggested to treat meibomian gland dysfunction. One such treatment is disclosed in U.S. Patent Application Publication No. 2003/0114426 entitled “Method for Treating Meibomian Gland Disease,” U.S. Pat. No. 6,455,583 entitled “Method for Treating Meibomian Gland Disease” to Pflugfelder et al., and PCT Publication Application No. WO 99/58131 entitled “Use of Tetracyclines for Treating Meibomian Gland Disease.” However, this treatment has not proven to be universally clinically effective, and it may be unnecessary in cases where MGD is the result of obstruction of the gland without infection.
The use of corticosteroids has also been proposed to treat MGD as disclosed in U.S. Pat. No. 6,153,607 entitled “Non-preserved Topical Corticosteroid for Treatment of Dry Eye, filamentary Keratitis, and Delayed Tear Clearance (or Turnover)” to Pflugfelder et al. Again, this proposed treatment appears to treat the symptoms of dry eye, as opposed to treatment of the underlying cause.
Additionally, the use of topically applied androgens or androgen analogues has also been used to treat acute dry eye signs and symptoms in keratoconjuctivitis sicca. This is disclosed in U.S. Pat. Nos. 5,958,912 and 6,107,289, both entitled “Ocular Therapy in Keratoconjunctivitis Sicca Using Topically Applied Androgens or TGF-beta.” and both issued to Sullivan.
There is a correlation between the tear film lipid layer and dry eye disease. The various different medical conditions and damage to the eye and the relationship of the lipid layer to those conditions are reviewed in Surv Opthalmol 52:369-374, 2007. It is clear that the lipid layer condition has the greatest effect on dry eye disease when compared to the aqueous layer or other causes. Thus, while dry eye states have many etiologies, the inability of the meibomian gland 10 to sufficiently generate the lipid layer is a common cause of common dry eye state. This state is the condition known as “meibomian gland dysfunction” (MGD). MGD is a disorder where the meibomian glands 10 are obstructed or occluded. As employed herein the terms “occluded” and “obstruction” as they relate to meibomian gland dysfunction are defined as partially or completely blocked or plugged meibomian glands. If completely obstructed the gland cannot secrete. If partially or intermittently occluded the gland may secrete either normal or decreased amounts of sebum. More usually the secretions are altered having semi-solid, thickened, congested secretions, frequently described as inspissated. The secretions may be clear or yellowish, the latter indicating possible infection. Meibomitis, an inflammation of the meibomian glands leading to their dysfunction, is usually accompanied by blepharitis (inflammation of the lids). Meibomian gland dysfunction may accompany meibomitis, or meibomian gland dysfunction may be present without obvious lid inflammation.
MGD is frequently the result of keratotic obstructions, which partially or completely block the meibomian gland orifices 16. Such obstructions compromise the secretory functions of the individual meibomian glands 10. More particularly, these keratotic obstructions may be associated with or result in various combinations of bacteria, sebaceous ground substance, dead, and/or desquamated epithelial cells (see, Meibomian Gland Dysfunction and Contact Lens Intolerance, Journal of the Optometric Association, Vol. 51, No. 3, Korb et al., (1980), pp. 243-51).
Hormonal changes, which occur during menopause and particularly changing estrogen levels, can result in thickening of the oils secreted by the meibomian glands 10. This may result in clogged gland orifices. Further, decreased estrogen levels may also enhance conditions under which staphylococcal bacteria can proliferate. This can cause migration of the bacteria into the glands 10 compromising glandular function and further contributing to occlusion, thus resulting in a decreased secretion rate of the meibomian gland 10.
When the flow of secretions from the meibomian gland 10 is restricted due to the existence of an occlusion, cells on the eyelid margin have been observed to grow over the gland orifice 16. This may further restrict sebum flow and exacerbate a dry eye condition. Additional factors may also cause or exacerbate meibomian gland dysfunction including age, disorders of blinking, activities such as computer use which compromise normal blinking, contact lens use, contact lens hygiene, cosmetic use, or other illness, particularly diabetes. The state of an individual meibomian gland 10 can vary from optimal, where clear meibomian fluid is produced; to mild or moderate meibomian gland dysfunction where milky fluid or inspis sated or creamy secretion is produced; to total blockage, where no secretion of any sort can be obtained (see “Increase in Tear Film Lipid Layer Thickness Following Treatment of Meibomian Gland Dysfunction,” Lacrimal Gland, Tear Film, and Dry Eye Syndromes,” Korb, et al., pp. 293-98, Edited by D. A. Sullivan, Plenum Press, New York (1994)). Significant chemical changes of the meibomian gland 10 secretions occur with meibomian gland dysfunction and consequently, the composition of the naturally occurring tear film is altered, which in turn, contributes to dry eye.
MGD may be difficult to diagnose, because visible indicators are not always present. For example, meibomitis, an inflammation of the meibomian glands 10, can lead to MGD. Meibomitis may also be accompanied by blepharitis (inflammation of the lids). While meibomitis is obvious by inspection of the external lids, MGD may not be obvious even when examined with the magnification of the slit-lamp biomicroscope. This is because there may not be external signs or the external signs may be so minimal that they are overlooked. The external signs of MGD without obvious lid inflammation may be limited to subtle alterations of the meibomian gland orifices 16, overgrowth of epithelium over the orifices 16, and pouting of the orifices 16 of the glands 10 with congealed material acting as obstructions. In severe instances of MGD without obvious lid inflammation, the changes may be obvious, including serrated or undulated lid margins, orifice recession and more obvious overgrowth of epithelium over the orifices 16, and pouting of the orifices 16.
Thus to summarize, the meibomian glands 10 of mammalian (e.g., human) eyelids secrete oils that prevent evaporation of the tear film and provide lubrication to the eye and eyelids. These glands can become blocked or plugged (occluded) by various mechanisms leading to so-called “dry eye syndrome.” While not the only cause, MGD is a known cause of dry eye syndrome. The disorder is characterized by a blockage of some sort at an orifice of the meibomian glands 10 preventing normal lipid secretions from flowing from the meibomian glands 10 to form the lipid layer of the tear film. Such secretions serve to prevent evaporation of the aqueous tear film and lubricate the eye and eyelids 12, 14, hence, their absence can cause dry eye syndrome.
While the present state of the art provides a number of treatments for dry eye, there is a need to treat the underlying cause, as opposed to the symptom. Many patients suffer from dry eye as a result of obstructions or occlusions in the meibomian glands. Thus, a need exists to provide effective treatment of the meibomian glands to restore a sufficient flow of sebum to the lipid layer of the eye to limit the rate of evaporation of the underlying layers.